The morbidity and mortality of HIV infection are largely due to immunosuppression. Abnormalities in cell-mediated immunity appear to be of greatest clinical import. Such abnormalities include: (1) aberrant or absent CD4+ T cell responses; (2) inefficient CD8+ T cell activity; and (3) dysregulation of antigen-presenting cell (APC) function, including a defect in the production of interleukin-12 (IL-12), a mediator central to the induction and maintenance of cell-mediated immunity. CD4+ T cell help is essential for optimal APC and CD8+ T cell function. Interactions between CD40 (on APC) and CD40Ligand [CD40L] (on activated CD4+ T cells) are crucial for such CD4 help. CD40L upregulation becomes progressively impaired in HIV infection, in parallel with increasing immunosuppression. Further, HIV-related defects in IL-12 production and CD8+ T cell function can be ameliorated in vitro with exogenous CD40L. Finally, people with genetic mutations in CD40L suffer from similar opportunistic infections as individuals with HIV/AIDS. The central hypothesis underlying this application is that defective upregulation of CD40L expression on CD4+ T cells is central to the suppression of cell-mediated immunity seen in HIV infection. Our preliminary studies of mechanism suggest that: (1) CD40L dysregulation occurs at level of the signaling cascades upstream of the CD40L promoter, specifically in the steps immediately following TCR engagement; and (2) engagement of CD4 by the HIV gp120 envelope protein (or HTV virions themselves) prior to T cell activation, models the CD40L dysregulation seen in CD4+ T cells from HIV-infected donors. Preliminary data from a longitudinal study of patients starting highly-active anti-retroviral therapy also suggest that treatment-mediated viral suppression leads to amelioration of this defect in CD40L expression. The long-term goal of this research is to apply mechanistic insights into HIV immunopathogenesis to the development of novel therapeutic approaches to HIV. The goal of this application is to define the mechanisms underlying CD40L dysregulation in HIV. We aim to examine the effect of HTV on CD40L induction by determining (1) the functional consequences of blunted CD40L, with a special emphasis on the defective maturation signals given to dendritic cells; (2) the key signaling mechanisms which are target(s) of CD4 pre-engagement; and (3) the impact of therapeutic in vivo viral suppression on CD40L regulation and CD40L-dependent cell-mediated immunity, through longitudinal studies of individuals with chronic HIV infection.